Generally, steel must be heated to approximately 1,250° C. for a long time in a heating furnace before hot-rolling of the steel may be carried out. The protracted heating process results in serious oxidation loss to the steel surface. That is, in the absence of antioxidation measures, a thick scale of iron oxide forms on the surface of the steel. This scale must be removed from the surface of the steel using a sometimes uneven process. Commonly, the loss rate due to oxidation during the heating process can reach up to 0.5%˜2.5%. This not only reduces productivity in the processing of the steel, but affects the surface quality of steel.
Applying an antioxidation coating to the steel surface to reduce the oxidation loss forms a protective layer on the steel surface by which oxidation/decarbonization of the steel surface can be prevented during the heating process. During the subsequent cooling process, the protective layer is peeled off to decrease the thickness of the iron scale, thereby increasing productivity and improving the quality of the steel surface.
For effective reduction of oxidation loss, the antioxidation coating must form a complete and dense protective layer that must be adaptable the high-temperatures in the heating furnace and then can be peeled off easily along with the iron scale after the steel is discharged from the furnace. Furthermore, it is desirable to reduce the thickness of the iron scale as much as possible so that metallic luster of the steel surface can be retained after heat treatment.
There have been many reports on the research about this type of antioxidation coating, for example, Chinese Patent CN1632137A describes a protective coating material that could be used to prevent oxidation/decarbonization of the steel workpiece during heat treatment. The coating is composed of: glass powder 10%˜20%, quartz sand 30%˜33%, silica sand 20%˜23%, Al2O3 20%˜24%, and C 2%˜10%. The coating is applied to effect antioxidation/decarbonization of various types of steel heated to 800˜1200° C. for 4˜10 hours and is able to peel spontaneously during a subsequent cooling process. Accordingly, the surface of the steel workpiece maintains its original metallic luster after heat treatment, and the surface layer also maintains its original chemical components. According to the description of CN1632137A, this coating material is convenient to use and has excellent antioxidation/decarbonization effectiveness and wide applicability. However, this coating material has limited application because the coating material must be applied at a temperature lower than 1200° C. Thus, the operation of the heating furnace is limited during the use of this coating. That is, in order to apply this coating material, it is essential that the hot steel be cooled previously, then coated with the coating material, and afterwards re-heated for hot-rolling. Such additional processing consumes both additional energy and time.
Chinese Patent CN1300805A describes a high-temperature antioxidation coating prepared by mixing 50˜62 parts of SiO2, 12˜20 parts of Al2O3, 2˜8 parts of Fe2O3, 1˜5 parts of MgO, 2˜7 parts of CaO, 2˜6 parts of Na2O, 2˜6 parts of K2O, and 1˜5 parts of FeO. The mixture is then melted in a kiln at 800˜1300° C. for 3 hours to form an enamel that is then cooled. The cooled enamel is then pulverized with the addition of a small amount of spinel, clay, and water, and finally ground to obtain a particle size of less than 180 mesh. Such coating particles are fine and in a form of a suspension, thus hardly depositing; at the same time, the coating is simple to use without requiring vertical roll side-pressure equipment, can be used at a temperatures of 900˜1200° C. and is low cost. However, the technology and processing to produce this coating is rather complicated because the coating components first must be melted to form enamel and then cooled and pulverized, leading to time and energy consumption. Furthermore, the application temperature of this coating is lower than 1200° C. so that the coating cannot be directly applied at the heat treatment temperature before the common steel is hot rolled.
Chinese Patent CN1179448A describes a high temperature refractory antioxidation coating for steel that mainly consists of precipitated barium carbonate and silica, in addition to boric acid as fluxing agent and soluble glass as medium. Under the high temperatures, this coating material forms a continuous ceramic glaze which has excellent spreading properties with no stripping, no gasification and no dropping, and good antioxidation protection to prevent the refractory steel from oxidizing in long-term use. However, this coating is oriented specifically at protecting refractory steel from oxidizing at high temperatures, and is used for a surface of the refractory steel for smelting metal magnesium. Moreover, the coating is used with heating only when it is sprayed at normal temperature and then forms a dried film spontaneously, and thus does not have broad applicability.
Chinese Patent CN1036396A describes an antioxidation coating for MgO-Cr2O3-based silicon steel slab. The coating uses industrial magnesia as main raw material, is low cost and easy to produce without any cleaning treatment for the slab before using it, and has excellent thermal conductivity and high temperature resistance. The application of this coating does not result in an extend heating time and a negative effect on the finished product quality of the silicon steel, and can effectively prevent the oxidation loss of the silicon steel in heating; furthermore, the furnace throughput can be improved and the primary rolling capacity can be increased greatly. However, this coating is oriented specifically at protecting silicon steel from oxidizing at high temperatures, and thus has no broad applicability.
It can be seen that the current antioxidation coatings have good high temperature antioxidation effect on the certain steels to some extent, but each of the protection temperatures was at most 1200° C., and therefore the applications of these coatings were limited. Moreover, the steel must be cooled before the coating is applied, thereby increasing energy consumption. As far as the prior art, there is still room both to raise the coating temperature of the coating on the steel surface and to raise the protection temperature of the coating layer. In addition, glass powder-based substances are often used as the sticking medium in the present coatings in order to ensure the coating performance, but preparation of these glass powder-based substances requires complicated technology such as high temperature sintering, shredding and grinding to prepare such glass powders, complicating the initial preparation process of these coatings.